1. Field of the Invention
The present invention relates to a scanning optical device and an image forming apparatus using the device and, more particularly, to a scanning optical device which records image information by deflecting a light beam emitted from a light source means using a deflection element, and optically scanning a scanning target surface via an imaging element having an f-xcex8 characteristic and is suitable for an image forming apparatus using an electrophotographic process, such as a laser beam printer, digital copying machine, or multi-functional printer.
2. Related Background Art
Conventionally, in a scanning optical device used for a laser printer (LBP), digital copying machine, or the like, a light beam which is optically modulated in accordance with an image signal and emitted from a light source means is periodically deflected by a light deflector such as a rotating polyhedral mirror (polygon mirror), and the light is focused into a spot on the surface of a photosensitive recording medium (photosensitive drum) by a scanning optical element having an f-xcex8 characteristic, thereby optically scanning the surface and recording an image.
FIG. 14 is a schematic view of the main part of a conventional scanning optical device of this type.
Referring to FIG. 14, a divergent light beam emitted from a light source means 91 is converted into a substantially parallel light beam by a collimator lens 92. The light beam (light amount) is limited by a stop 93 and incident on a cylindrical lens 94 having a predetermined refracting power only in the sub scanning direction. Of the substantially parallel light beam incident on the cylindrical lens 94, the light in a main scanning cross-section emerges without any change. The light in a sub scanning cross-section is focused and substantially formed into a linear image on a deflection surface (reflection surface) 95a of a light deflector 95 formed by a rotating polyhedral mirror (polygon mirror).
The light beam reflected/deflected by the deflection surface 95a of the light deflector 95 is guided onto a photosensitive drum surface 98 as a scanning target surface via an imaging means (f-xcex8 lens) 96 having an f-xcex8 characteristic. By rotating the light deflector 95 in the direction indicated by an arrow A, the photosensitive drum surface 98 is optically scanned in the direction indicated by an arrow B. With this operation, an image is recorded on the photosensitive drum surface 98 as a recording medium.
A scanning optical device of this type needs to form a uniform spot by correcting the curvature of field throughout the scanning target surface and have a distortion (f-xcex8 characteristic) that makes the angle of incident light and image height have a proportional relationship.
Recently, however, in addition to these requirements, the following are considered important to realize high-resolution printing:
(1) to set a uniform spot size in the sub scanning direction within an image effective area; and
(2) to set a uniform adjacent pitch interval within the image effective area in multi-beam scanning. It is therefore required that the F-number (Fno) in the sub scanning direction with respect to a light beam incident on the scanning target surface be uniform within the image effective area.
As a scanning optical element, a plastic lens has become mainstream, which has the merit of realizing high-precision aberration correction using an aspherical surface, allowing mass production by molding at low cost, and the like.
A plastic lens, however, has the property of changing in refractive index with a use environment change (temperature change). For this reason, focus variation occurs on the scanning target surface, posing a problem in high-resolution printing. It is therefore required to perform temperature compensation in the sub scanning direction in which the imaging magnification is high and focus variation is noticeable.
According to Japanese Patent Application Laid-Open Nos. 8-297256 and 10-232347 and the like previously proposed by the present assignee, the radii of curvature of at least two surfaces of a scanning optical element in the sub scanning direction are changed from an on-axis portion toward an off-axis portion with respect to the main scanning direction to make the F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface uniform within the image effective area.
According to these references, the F-number is made uniform by bending the surface shape within a sub scanning cross-section. For this reason, the radius of curvature of a surface in the sub scanning direction may abruptly change depending on the surface shape within a main scanning cross-section of the scanning optical element.
It is an object of the present invention to provide a scanning optical device which can make the F-number (Fno) in the sub scanning direction with respect to a light beam incident on a scanning target surface uniform with an easy, simple arrangement by further improving the above proposals without impairing the environmental performance, and is suitable for high-resolution printing, and an image forming apparatus using the device.
In one aspect of the invention, scanning optical device includes:
a deflection element which reflects/deflects a light beam emitted from light source means in a main scanning direction; and
a scanning optical system which forms the light beam reflected/deflected by said deflection means into an image on a scanning target surface,
wherein said scanning optical system includes an optical element having at least one diffraction surface and at least one optical element having a refraction surface, and
a radius of curvature of at least one refraction surface in a sub scanning direction changes from an on-axis portion toward an off-axis portion, and a diffraction power of at least one diffraction surface in the sub scanning direction changes from an on-axis portion toward an off-axis portion.
In further aspect of the foregoing scanning optical device, the radius of curvature of said at least one refraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, the diffraction power of said at least one diffraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface is made substantially constant within an image effective area by changing the radius of curvature of said at least one refraction surface in the sub scanning direction and the diffraction power of said at least one diffraction surface in the sub scanning direction.
In further aspect of the foregoing scanning optical device, an image magnification of said scanning optical system in the sub scanning direction is substantially constant within an image effective area.
In further aspect of the foregoing scanning optical device, letting Fmax and Fmin be maximum and minimum values of an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface within an image effective area, the maximum and minimum values Fmax and Fmin satisfy
Fmin/Fmax greater than 0.9
In further aspect of the foregoing scanning optical device, a change in focus of said scanning optical device on the scanning target surface in the sub scanning direction due to an environmental variation is compensated for by changes in power of the refraction surface and diffraction surface of said scanning optical system and a change in wavelength of said light source means.
In further aspect of the foregoing scanning optical device, letting xcfx86Sd be power of the diffraction surface of said scanning optical system on an optical axis in the sub scanning direction, and xcfx86Sr be power of the diffraction surface on the optical axis in the sub scanning direction, the powers xcfx86Sd and xcfx86Sr satisfy
1.0 less than xcfx86Sr/xcfx86Sd less than 2.6
In further aspect of the foregoing scanning optical device, at least one of a change in the radius of curvature of the refraction surface of said scanning optical system in the sub scanning direction and a change in the diffraction power of the diffraction surface in the sub scanning direction is asymmetrical with respect to the optical axis of said scanning optical system.
In further aspect of the foregoing scanning optical device, at least one optical element of said scanning optical system is manufactured by plastic molding.
In another aspect of the invention, a scanning optical device includes:
light source means for emitting a plurality of light beams which are optically modulated independently;
a deflection element which reflects/deflects a plurality of light beams in a main scanning direction; and
a scanning optical system which forms the plurality of light beams reflected/deflected by said deflection means into an image on a scanning target surface,
wherein said scanning optical system includes an optical element having at least one diffraction surface and at least one optical element having a refraction surface, and
a radius of curvature of at least one refraction surface in a sub scanning direction continuously changes from an on-axis portion toward an off-axis portion, and a diffraction power of at least one diffraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion.
In further aspect of the foregoing scanning optical device, the radius of curvature of said at least one refraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, the diffraction power of said at least one diffraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface is made substantially constant within an image effective area by changing the radius of curvature of said at least one refraction surface in the sub scanning direction and the diffraction power of said at least one diffraction surface in the sub scanning direction.
In further aspect of the foregoing scanning optical device, an image magnification of said scanning optical system in the sub scanning direction is substantially constant within an image effective area.
In further aspect of the foregoing scanning optical device, letting Fmax and Fmin be maximum and minimum values of an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface within an image effective area, the maximum and minimum values Fmax and Fmin satisfy
xe2x80x83Fmin/Fmax greater than 0.9
In further aspect of the foregoing scanning optical device, a change in focus of said scanning optical device on the scanning target surface in the sub scanning direction due to an environmental variation is compensated for by changes in power of the refraction surface and diffraction surface of said scanning optical system and a change in wavelength of said light source means.
In further aspect of the foregoing scanning optical device, letting xcfx86Sd be power of the diffraction surface of said scanning optical system on an optical axis in the sub scanning direction, and xcfx86Sr be power of the diffraction surface on the optical axis in the sub scanning direction, the powers xcfx86Sd and xcfx86Sr satisfy
1.0 less than xcfx86Sr/xcfx86Sd less than 2.6
In further aspect of the foregoing scanning optical device, at least one of a change in the radius of curvature of the refraction surface of said scanning optical system in the sub scanning direction and a change in the diffraction power of the diffraction surface in the sub scanning direction is asymmetrical with respect to the optical axis of said scanning optical system.
In further aspect of the foregoing scanning optical device, at least one optical element of said scanning optical system is manufactured by plastic molding.
In still another aspect of the invention, a scanning optical device includes:
a deflection element which reflects/deflects a light beam emitted from light source means in a main scanning direction; and
a scanning optical system which forms the light beam reflected/deflected by said deflection means into an image on a scanning target surface,
wherein said scanning optical system includes a single optical element having refraction and diffraction surfaces, and
a radius of curvature of the refraction surface in the sub scanning direction changes from an on-axis portion toward an off-axis portion, and a diffraction power of the diffraction surface in the sub scanning direction changes from an on-axis portion toward an off-axis portion.
In further aspect of the foregoing scanning optical device, the radius of curvature of the refraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, the diffraction power of the diffraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface is made substantially constant within an image effective area by changing the radius of curvature of the refraction surface in the sub scanning direction and the diffraction power of the diffraction surface in the sub scanning direction.
In further aspect of the foregoing scanning optical device, an image magnification of said scanning optical system in the sub scanning direction is substantially constant within an image effective area.
In further aspect of the foregoing scanning optical device, letting Fmax and Fmin be maximum and minimum values of an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface within an image effective area, the maximum and minimum values Fmax and Fmin satisfy
Fmin/Fmax greater than 0.9
In further aspect of the foregoing scanning optical device, a change in focus of said scanning optical device on the scanning target surface in the sub scanning direction due to an environmental variation is compensated for by changes in power of the refraction surface and diffraction surface of said scanning optical system and a change in wavelength of said light source means.
In further aspect of the foregoing scanning optical device, letting xcfx86Sd be power of the diffraction surface of said scanning optical system on an optical axis in the sub scanning direction, and xcfx86Sr be power of the diffraction surface on the optical axis in the sub scanning direction, the powers xcfx86Sd and xcfx86Sr satisfy
1.0 less than xcfx86Sr/xcfx86Sd less than 2.6
In further aspect of the foregoing scanning optical device, one of a change in the radius of curvature of the refraction surface of said scanning optical system in the sub scanning direction and a change in the diffraction power of the diffraction surface in the sub scanning direction is asymmetrical with respect to the optical axis of said scanning optical system.
In further aspect of the foregoing scanning optical device, one optical element of said scanning optical system is manufactured by plastic molding.
In still another aspect of the invention, a scanning optical device includes:
light source means for emitting a plurality of light beams which are optically modulated independently;
a deflection element which reflects/deflects a plurality of light beams in a main scanning direction; and
a scanning optical system which forms the plurality of light beams reflected/deflected by said deflection means into an image on a scanning target surface,
wherein said scanning optical system includes a single optical element having refraction and diffraction surfaces, and
a radius of curvature of the refraction surface in the sub scanning direction changes from an on-axis portion toward an off-axis portion, and a diffraction power of the diffraction surface in the sub scanning direction changes from an on-axis portion toward an off-axis portion.
In further aspect of the foregoing scanning optical device, the radius of curvature of the refraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, the diffraction power of the diffraction surface in the sub scanning direction continuously changes from an on-axis portion toward an off-axis portion with respect to the main scanning direction.
In further aspect of the foregoing scanning optical device, an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface is made substantially constant within an image effective area by changing the radius of curvature of the refraction surface in the sub scanning direction and the diffraction power of the diffraction surface in the sub scanning direction.
In further aspect of the foregoing scanning optical device, an image magnification of said scanning optical system in the sub scanning direction is substantially constant within an image effective area.
In further aspect of the foregoing scanning optical device, letting Fmax and Fmin be maximum and minimum values of an F-number in the sub scanning direction with respect to a light beam incident on the scanning target surface within an image effective area, the maximum and minimum values Fmax and Fmin satisfy
Fmin/Fmax greater than 0.9
In further aspect of the foregoing scanning optical device, a change in focus of said scanning optical device on the scanning target surface in the sub scanning direction due to an environmental variation is compensated for by changes in power of the refraction surface and diffraction surface of said scanning optical system and a change in wavelength of said light source means.
In further aspect of the foregoing scanning optical device, letting xcfx86Sd be power of the diffraction surface of said scanning optical system on an optical axis in the sub scanning direction, and xcfx86Sr be power of the diffraction surface on the optical axis in the sub scanning direction, the powers xcfx86Sd and xcfx86Sr satisfy
1.0 less than xcfx86Sr/xcfx86Sd less than 2.6
In further aspect of the foregoing scanning optical device, one of a change in the radius of curvature of the refraction surface of said scanning optical system in the sub scanning direction and a change in the diffraction power of the diffraction surface in the sub scanning direction is asymmetrical with respect to the optical axis of said scanning optical system.
In further aspect of the foregoing scanning optical device, one optical element of said scanning optical system is manufactured by plastic molding.
In still another aspect of the invention, an image forming apparatus includes the foregoing scanning optical device, a photosensitive member placed on the scanning target surface, a developing unit which develops an electrostatic latent image formed on said photosensitive member by a light beam scanned by said scanning optical device into a toner image, a transfer unit which transfers the developed toner image onto a transfer material, and a fixing unit which fixes the transferred toner image on the transfer material.
In still another aspect of the invention, an image forming apparatus includes the foregoing scanning optical device, and a printer controller which converts code data input from an external unit into an image signal and inputs the signal to said scanning optical device.